Tetravalent chemistry ligand complexes

Coordination Complexes. The coordination and organometaHic chemistry of thorium is dominated by the extremely stable tetravalent ion. Except in a few cases where large and stericaHy demanding ligands are used, lower thorium oxidation states are generally unstable. An example is the isolation of a molecular Th(III) complex [107040-62-0] Th[Tj-C H2(Si(CH2)3)2]3 (25). Reports (26) on the synthesis of soluble Th(II) complexes, such as... 

Sulfur, thiolates and sulfide ligands form very stable complexes with platinum. Many complexes have platinum in the divalent state, but complexes with a Pt—S bond are formed with platinum in the zerovalent or tetravalent state. Several recent reviews have been written on various aspects of the platinum coordination chemistry of sulfur heteroatom ligands, and these are listed in Table 9. 

Also present in many natural waters are humic/fulvic acid, citric acid, and the like. These organics also can complex actinides. In Figure 15.18, we show the relative stability constants for the first complexation reaction of various ligands with actinides of different oxidation states. Clearly, the carbonate and humate ions along with hydrolysis dominate the chemistry. The tetravalent actinide ions will tend toward hydrolysis reactions or carbonate complexation rather than humate/fulvate formation. 

As ligands for osmium, ammonia and ethylenediamine (en 1,2-diaminoethane) would be expected to have broadly similar properties, and this is on the whole true — there are stable osmium(III) complexes such as [Os(en)3]3+, trans-[Os(en)2 X2]+ (X — Cl, Br) and [Os(en)Cl4], and more recently the tetravalent Os(en)X4 (X = Cl, Br, I) has been made. There is, however, an interesting chemistry of the l,2-ethanediaminato-(l —) and -(2 —) ligands with osmium in the higher IV, V and VI states, and another unusual feature is the existence of the hydride [OsH2(en)2f. ... 

In aqueous solution, the chemistry of tetravalent vanadium is centered around the V02+ ion. This ion forms strong complexes with a diversity of ligands and is known to bind to numerous proteins21. The coordination chemistry of V02+, which is relevant to its biochemistry, has been recently reviewed by the author21. Five and six coordinate V02+ complexes are formed in which the short vanadyl VO bond length of —0.16 nm is... 

The chemistry of actinide ions is generally determined by their oxidation states. The trivalent, tetravalent and hexavalent oxidation states are strongly complexed by numerous naturally occurring ligands (carbonates, humates, hydroxide) and man-made complexants (like EDTA), moderately complexed by sulfate and fluoride, and weakly complexed by chloride (7). Under environmental conditions, most uncomplexed metal ions are sorbed on surfaces (2), but the formation of soluble complexes can impede this process. With the exception of thorium, which exists exclusively in the tetravalent oxidation state under relevant conditions, the dominant solution phase species for the early actinides are the pentavalent and hexavalent oxidation states. The transplutonium actinides exist only in the trivalent state under environmentally relevant conditions. 

The nonaqneons chemistry of the tri- and tetravalent ions follows similar trends as the aqueous complexes. Coordination numbers are dictated by the steric bulk and electronic properties of the ligands two stmctrrres, U(MeBH3)4 (11) and [ U(N(CH2CH2NSiMe3)3) 2(M - 7 7 -N2)] (12), are showtt Direct comparisorts of the latter species can be made to the behavior of transition metals in both coordination and reactivity. 

Chromium can exist in several oxidation states from Cr(0), the metallic form, to Cr(Vl). The most stable oxidation states of chromium in the environment are Cr(lll) and Cr(Vl). Besides the elemental metallic form, which is extensively used in alloys, chromium has three important valence forms. The trivalent chromic (Cr(lll)) and the tetravalent dichromate (Cr(Vl)) are the most important forms in the environmental chemistry of soils and waters. The presence of chromium (Cr(Vl)) is of particular importance because in this oxidation state Cr is water soluble and extremely toxic. The solubility and potential toxicity of chromium that enters wetlands and aquatic systems are governed to a large extent by the oxidation-reduction reactions. In addition to the oxidation status of the chromium ions, a variety of soil/sediment biogeochemical processes such as redox reactions, precipitation, sorption, and complexation to organic ligands can determine the fate of chromium entering a wetland environment. 

Now a series of tetravalent germanium complexes of this ligand has been synthesized and at the same time the series of germanium phthalocyanines (3,4) has been extended. Accordingly comparisons between the two series have been made possible and these enable the chemistry of octahedral germanium to be more clearly delineated. 

Siloxide ligands are able to coordinate to rare earth metals in various oxidation states and coordination numbers to primarily form mono- and dinuclear complexes. In particular, the synthetic and stmctural chemistry of trivalent rare earth siloxides are well documented in the literature and show analogies with rare earth alkoxides. It is fair to state, however, that the field of divalent and tetravalent rare earth siloxides is poorly developed and that applications pertaining to the design of siloxide-based homogeneous and heterogeneous rare earth metal catalysts as well as the development of novel silicate-based materials are scarce. Although the few results of the catalytic activity of some of the rare earth siloxides in olefin... 

The chemistry of rare earths is often discussed only in terms of the trivalent ions and indeed, contrary to the actinides, the oxidation states encountered in lanthanide compounds in the solid state and especially in solution are few in number. Standard electrode potentials M(II-III) and M(III-IV) indicate that, besides the trivalent rare earth ions, only Eu (-0.35 V), Yb + ( — 1.15 V), Sm + ( — 1.55 V) and Ce (+1.74 V) are sufficiently stable to exist in aqueous solutions (Nugent, 1975). It has long been known that alkaline conditions and many complexing anions such as nitrate, phosphate and sulfate stabilize Ce(IV) (Jorgensen, 1979) and recently it has been shown that large complex-forming ligands such as heteropolyanions also stabilize to some extent tetravalent praseodymium and terbium (Spitsyn, 1977). 

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